24 May 2017

Doppler spread monitoring in 9 MHz band signals

Looking for "Spectan" software download I come across an interesting web page about the "Precision Carrier Doppler Analysis": intrigued by this argument, I tried to replicate the Doppler spread analysys and these are the results of my one-day monitoring of two transmissions in the 9 MHz band (9182.0 and 9115.0 KHz, both in USB).

Due to the time-varying nature of the ionosphere, the propagation path is never static and the received sky-wave signals may suffer distortion in the form of temporal dispersion (delay spread) as well as fluctuation in the signal’s amplitude and phase (Doppler spreading). Recent high latitude measurements have observed multipath signals of more than 10 ms duration and other signals have shown evidence of Doppler spreading greater than 50 Hz. More typical mid-latitude sky-wave channels might show delay spreads of 1 - 4 ms with Doppler spreads of 1 Hz or less.In a few words, Doppler spread occur because during the day the apparent height at which signals are reflected changes quite markedly, leading to quite easily observable frequency shifts. It is the rate of change of apparent height which is related to the frequency shift. Doppler spread is commonly defined as the range of frequencies over which the received Doppler spectrum is essentially non-zero. When a pure sinusoidal tone of frequency fc is transmitted, the received signal spectrum will have components in the range fc – fd to fc + fd , where fd is the Doppler shift.

Looking for suitable transmissions to monitor, I decided in favor of the continuous B'casts of the Russian Navy on 9 MHz band: such transmissions are on USB and use the AT-3004D modem known as CIS-12. The signal consists of 12 BPSK modulated tones (MPSK), 120 bps per channel, with a Pilot Tone at ~3300 Hz which is just used for Doppler correction at receiving sites.

1) daylight path, 9182.0 KHz CIS-12 transmission (Fig. 1)

Fig. 1

During the daylight path the the Doppler spread is less than 1 Hz (as expected), since during the day the D layer supposedly absorbs the signal before it reaches the ionosphere. However, the absorbtion is not always complete, and the signal is also propagted via its E-layer daytime reflection. The E layer is relatively stable, and shows little Doppler spread (Fig. 2).Starting from about 1630 UTC (Fig. 3), the region of the transmitter enters in its Grey Line and the signal starts to be seen from various scatter paths and then reflected from the F-layer. The rise of the Doppler spread is quite easily observable.

Starting from about 17.30-1800 UTC (summer time, in my area) the D layer stops absorbing completely, and the signal starts to be reflected from the F-layer. At this time the effective height of the F layer is rising as ion density decreases and the Doppler spread reflects the instability of the F layer. I do not know the reason of the drift around 20.00 UTC.

Fig. 5

Fig. 6

It's interesting to see that the two transmissions have Doppler tones which differ of about 10 Hz: most likely it is due to two different transmitters.

3) setup

As said, the software used for this monitoring is "Spectran" - Current
version : Version 2 build 216 - and it can be downloaded from http://www.weaksignals.com/

Spectran is a spectrum analyzer written by Alberto, I2PHD and Vittorio, IK2CZL, members of the PAcket Digital Amateur Network group (PADAN), who created also other weak signal and QRSS programs. Spectran allows real time or deferred spectral analysis / waterfall display, in addition to real time audio filtering (band pass, denoising, band reject and CW peaking) of audio signals, using the PC sound card to digitize the input analog signal, or taking as input a WAV file. Its characteristics are well suited to dig weak signals buried into noise, thanks to a selectable bin size down to 21 millihertz.

The "Doppler mode" settings that I used for this monitoring are shown in Figure 7:

Fig. 7

And... yes, It would be much more interesting to monitor the same transmission for more than one day and in different seasons, but this is not my job :)

Antonio, very interesting material!On this topic I will post a couple of links, there guys from the Czech Republic are engaged in similar experiments. What's important is that the transmitters have a stable frequency! In my opinion, it is just as important to use a receiver with a high-stable reference oscillator when carrying out similar experiments. http://ok0eu.fud.cz/ http://ok1dub.cz/ok0eu/monitoring_ok0eu.htm

thanks for the comment Daniel. I used an SDR Receiver (Elad FDM-S1)and I chosed a supposed stable (militar) transmission, not a commercial broadcasting. I do not know the accuracy in ppm of that transmitter, anyway the difference of the spread is quite visible during the two different monitoring periods.